There is only one Superman. One Wonder Woman, one Captain America, one Ironman. All told, a few dozen superheroes. And billions of superhero fans.
What about our Solar System? Are we one-in-a-million like Batman? Or a dime-a-dozen like Batman’s fans? How unique, how special is our Solar System? For the first time in human history, we now can answer that question. We can even put numbers on it.
When we try to put ourselves in a broader context we need to keep in mind what we look like to others, what the Solar System would look like if we searched for it orbiting another star. Imagine an alien living on a planet orbiting a star 10 light years away from the Sun (in Galactic terms, right next door). After a decade or two spent searching for planets orbiting the Sun, this is what those aliens would find:
Pretty sparse, isn’t it? No Earth or Mars. No rocky planets at all! No Saturn, no asteroid belt, no ice giants. All the aliens could detect is Jupiter. And, just like those aliens, all that we would detect of the Solar System around another star would be the Sun and Jupiter. We could pinpoint Jupiter’s mass and orbit with decent precision, but that’s all.
Our question then becomes, how common are Sun-Jupiter systems?
Let’s start with the Sun. It turns out that our home star is a little bit unusual but not a lot. This image shows a census of all the stars within 30 light years of the Sun:
Our Sun is a G star. There are 20 G stars within 30 light years out of almost 400 total stars. The vast majority of stars are M stars, also known as “red dwarfs”. These small red stars have much longer lifetimes than G stars but shine much fainter. Among nearby stars, the Sun is modestly weird. If give our definition of a “Sun-like” star some latitude, our star ends up being rare at the 10% level. That is about the fraction of American adults who are vegetarian.
Now let’s turn our attention to Jupiter. To get started, let’s take a look at our current census of extra-solar planets. Here is a plot containing the almost 3,000 planets that have been discovered in the past two decades:
None of the known extra-solar line up with our rocky planets, or with Saturn, Uranus or Neptune. But we already knew this; it’s why we are searching for Sun-Jupiter systems and not complete Solar Systems.
Like the Sun, Jupiters are just a little out of the ordinary. Only 10-15% of Sun-like stars have a gas giant planet like Jupiter with a mass larger than about 50 times Earth’s mass. Another factor that makes our Jupiter more unusual is its orbit. Our Jupiter’s orbit is nearly circular (to be precise, it is an ellipse with an eccentricity of just 5%) and it is more than 5 times larger than Earth’s orbit. Only about 10% of the known gas giant extra-solar planets have orbits wider than Mars’ (which is only 1.5 times larger than Earth’s) that are nearly circular (with eccentricities less than 10%). Putting those together, only about one in a hundred stars like the Sun has a Jupiter like ours. That is a little less likely than being dealt three of a kind in a 5-card poker hand.
There is one more aspect of the Solar System that is unusual. It’s not something about the planets that we have; it’s something that is missing. About half of all stars like the Sun are orbited by at least one “hot super-Earth” planet. These planets are generally Earth-sized or larger, with orbits smaller than Mercury’s around the Sun. We don’t know why the Solar System is deficient in super-Earths, although one idea suggests that Jupiter may be the reason.
Let’s put the pieces together to calculate how unusual our Solar System is:
10% (the fraction of stars that are like the Sun)
x 10% (the fraction of Sun-like stars with Jupiters)
x 10% (the fraction of Jupiter-like planets with Jupiter-like orbits)
x 50% (the fraction of Sun-like stars with no hot super-Earths)
= 0.05% (1 in 2000)
This tells us that about one in every 2000 stars in our Galactic neighborhood is a Sun-Jupiter system. Those are about the odds of being picked if you apply to NASA to be an astronaut. But what does this mean? Well, our Solar System is definitely not rare enough to be Batman. Otherwise there would be thousands of Bat-women and men in every big city. It may be unusual enough for a lesser superpower like being able to eat ten hot dogs in one sitting, but not enough for the kind of superpower we like to talk about.
What this does mean is that the Solar System is not a dime-a-dozen kind of planetary system. Most systems are different than ours. Most planetary systems orbit puny red dwarfs instead of big yellow G stars like the Sun. Most systems contain super-Earths close to their stars, and if they happen to have a gas giant it tends to be much closer to its Sun or have a much more stretched out orbit than Jupiter’s.
But we don’t know how many Sun-Jupiter systems also have Venuses, Earths, Saturns or Marses. And we don’t know how common Earths and Saturns are in systems without Jupiters or systems that orbit red dwarfs. We really want to know whether there is a reason for life-bearing planets to prefer Sun-Jupiter systems. How exactly does the structure of a planetary system affect its ability to host life, if at all? We are still looking for the answers to those questions. And, who knows, maybe our Solar System will end up being Batman after all.
This blog post was the basis for an article I wrote for Nautil.us’s blog — see here.